Recording spectrometer



June 15, 1954 SAVlTZKY r 2,680,989

RECORDING SPECTROMETER Filed Dec. 22. 1950 3 Sheets-Sheet l Q Qt . II Ex gya fizz ATTORNEY June 15, 1954 A. SAVITZKY ETAL RECORDINGSPECTROMETER 3 Sheets-Sheet 2 Filed Dec. 22, 1950 VE OR ATTORNEYS f 4 WWJune 15, 1954 A. SAVITZKY ETAL 2,680,989

RECORDING SPECTROMETER Filed Dec. 22, 1950 3 Sheets-Sheet 3 m/W W 0 v Ty Patented June 15, 1954 RECORDING SPEGTROMETER Abraham Savitzky, Nor

S. Halford, New Your! Perkin-Elmer Corporation, corporation of New Yorkwalk, Conn, and Ralph N. Y., assignors to The Glenbrook, Conn, 2:.

Application December 22, 1950, Serial No. 202,276

(Cl. 88-1i) 14 Claims.

This invention relates to spectrometers, such as are commonly used tomeasure absorption spectra in the infrared range in analytical work.More particularly, the invention is concerned with a novel instrument,by means of which the ratio of the intensities of two different kinds ofradiation may be determined. A specific form of the new instrument maybe a spectrophotometer, in which a comparison may be readily madebetween spectra obtained by passing radiant energy through a cellcontaining a sample to be examined and a cell containing a reference orcomparison specimen. The new instrument is of relatively simpleconstruction and the comparison between the spectra is made in a singlerun. The instrument thus affords the advantages of both single anddouble beam spectrometers used for spectrophotometric work, whileavoiding their disadvantages. The instrument includes means forseparating two light signals out of phase by ap proximately 90 and suchmeans form part of the invention.

The instrument or" the invention may be employed in the infrared,visible, and ultraviolet ranges and is especially useful in the infraredregion. A form of embodiment of the invention utilizing radiation froman infrared source will, accordingly, be illustrated and described indetail for purposes of explanation. In the preferred form of such aninstrument, the ratio of the absorptions of the sample to be examinedand the comparison sample are automatically recorded throughout the run.

Spectrophotometers heretofore available fall into two classes dependingupon whether they make use or" a single beam of radiation or a doublebeam. In making a comparison of the spectra of a sample to be examinedand a comparison or reference specimen in a single beam instrument, tworuns are made in succession and these runs are intended to be identicalin all respects except that, in one run, the beam is intercepted by theunknown and, in the other, the unknown is replaced by the standard. Theratios of the absorption by the unknown and the standard are thencalculated point by point throughout the wavel ngths employed in theruns. The use of a single beam instrument for the purpose described isopen to serious objections, as, for example, the running time is oftenvery long. Also, the validity of the procedure is based on theassumption that the instrumental conditions in the two runs areidentical and it is extremely diificult to maintain constant thebrightness of the source, the temperature of the dispersing element, andother important factors. The introduction of errors in measurementresulting from variations in the conditions in the two runs can,accordingly, be avoided only by the utmost care.

Modifications in single beam instruments have heretofore been proposed,and such modifications have consisted primarily of the use of memorydevices for storing information in a no-sample or standard run, theinformation then being fed into the instrument during the sample run, sothat ratioing may be accomplished automatically and the ratios recorded.While such modifications would overcome some of the disadvantages ofsingle beam operation, they would not avoid "1e necessity of making tworuns with the possible errors resulting from changes in conditionsduring the runs.

instruments operating with a double beam, the sample and the comparisonare transilluminated simultaneously from the same source and as nearlyas possible through the same optics. The records of the two beams arethen available simultaneously and may be ratioed automatically in anumber of ways. One way commonly used is to weaken the comparison beamby motordriven devices, until it is equal to the sample beam, and thenrecord the weakening effort required. Such instruments are, however,very complex mechanically, optically, and electronically and, ingeneral, the strength of the unabsorbed beam is not available to operatedesired auxiliary devices.

The present invention is directed to the provision of a novelinstrument, which affords the advantages of simultaneous measurement ofthe comparison and sample absorption spectra and the ratioing of themeasurements. The new instrument is comparable in simplicity to a singlebeam instrument of ordinary construction and, in addition, it makesavailable the strength of an unabsorbed beam for operation of auxiliarydevices.

In the preferred form of the new instrument, the radiation from thesource is passed to a pair of cells, one of which contains the sampleand the other the comparison. The radiation transmitted by the sampleand the comparison falls upon a detector and the radiation isinterrupted or chopped periodically. Each beam is interrupted in a cyclehaving an on period and an off period and the two cycles are displacedby a phase difference of 90. The detector responds to the radiationfalling thereon, as by generating a voltage, and the detector signalmade up of two components is amplified and the amplifier output ispassed to a pair of rectifying and filtering means. One of therectifying and filtering means operates approximately in phase with thefirst component of the signal and 90 out of phase with the secondcomponent, while the other rectifying and filtering means operatesapproximately in phase with the second component and 90 out of phasewith the first. Each rectifying and filtering means thus rectifies thecomponent of the signal, with which it is in phase, and rejects, thatis, produces no D. C. output from the other component. The two rectifiedsignals are proportional in amplitude to the radiation transmitted tothe detector through the two cells and thus proportional to theradiation transmitted by the sample and the comparison. The two cyclesof operation in the instrument each have an off period greater than 90and, since the cycles are out of phase by 90, the result is that thecycles have an off period in common. As a consequence, a zero point isprovided and the actual value of the two rectified signals can bedetermined. Once the signals are separated as described, they may beratioed and recorded as by being fed to a servo-amplifier operating themotor of a ratio recorder,

For a better understanding of the invention, reference may be made tothe accompanying drawings, in which Fig. 1 is a diagrammatic plan viewof the new instrument;

Fig. 2 is a View in front elevation of the disc used for chopping thebeam in the instrument;

Fig. 3 is a diagram of the electrical circuits;

Figs. la-4f, inclusive, are diagrams showing the mode, in which therectifying means function;

Fig. 5 is a diagrammatic plan View of another form of instrumentembodying the invention;

Fig. 6 is a view in perspective of features of the instrument of Fig. 5;

Fig. 7 is an end elevational view of the chopping discs used in the Fig.5 instrument; and

Fig. 8 is a diagrammatic plan view of another modified form of the newinstrument.

The instrument, as illustrated in Figs. 1 and 2, is for infraredoperation and it includes a source S of infrared radiation, the sourcebeing of any well known type, such as that known commercially as aglobar. Light from the source passes to a fiat mirror I i, which directsthe light to a spherical mirror i2, which focuses the light through thecell it upon an entrance slit iii of a monochromator. The light passingthrough the slit falls upon and is collimated by an off-axis paraboloidHi, from which the beam passes through a prism f It to be dispersed andthence to an adjustable fiat mirror 5?. The light returns from mirror llthrough the prism to be dispersed further and to the paraboloid it,which focuses the light upon an exit slit it of the monochromator, thelight being turned on its way to the slit by a diagonal mirror !8. Therays passing through the exit slit fall upon a fiat mirror 23 and aredirected thereby upon a focusing mirror 2!. Light is focused by mirror26 upon a detector 22 and, by adjustment of mirror ii, light indifferent narrow bands of wavelengths may be directed upon the detector.The detector is of the type, which produces a response to the radiationfalling thereon, and any well known detector appropriate for receivingthe radiation supplied by the source may be used. In the instrumentillustrated, the detector responds to the radiation falling thereon bygenerating a voltage.

The cells employed in the instrument are of conventional constructionand may take the form of a housing provided with a partition dividing itinto two cells [311, [3b for containing the sample to be examined andthe standard or comparison, respectively. The housing has windowstransparent to the radiation at its opposite ends and the partitionextends in the direction of travel of the radiation. The cells may besaid to be offset laterally, by which it is to be understood thatradiation passing through each cell falls upon the detector withoutpassing through the other cell.

The beam of radiation about to enter the cells is chopped by a disc 23mounted on the shaft of a motor 24. The disc is formed with two arcuateslots 23a, 23b lying offset radially, each slot in the disc shown havinga length of 180 and the slots having an overlap of The disc and cellsare so disposed relatively to one another that, in each revolution ofthe disc, light from the source passes first through one cell only, thenthrough both cells, then through the second cell only, and is then cutoff.

The voltage generated by the detector passes to an amplifier 25 and isthen rectified and filtered, as follows. The output of the amplifier isfed to the primaries 25a, 21a. of a pair of transformers 25, 22. Thesecondary 26b, 2% of each trans former has a grounded center tap and theends of secondary 25?) are connected to the fixed contacts or poles a, bof a breaker A, while the ends of secondary 21?) are connected to thefixed contacts or poles a, b of a breaker B. Breaker A has a movable arm28 engageable with its poles and breaker B has a similar movable arm asengageable with its poles. The arms 28, 29 are connected to respectivefilters 38, 3|, which remove undesired A. C. components from therectified signals. The filters are connected to a rationing recordingdevice, which, in the diagram, includes a potentiometer 32 and aresistor 33 connected to the respective filters and to a common ground.The slider 34 of the potentiometer carries a pen making a record on arecord strip and is actuated by a motor 35 supplied with current from aservoamplifier 36 of conventional construction connected to the sliderand to the output terminal of filter 3 I.

The movable arms 28, 28 of the breakers A, B are driven in synchronismwith the chopper disc 23 and are preferably actuated by the motor 2 1.The breakers operate in cycles in synchronism with the chopper disc 23-,as follows.

Breaker A is synchronized with the inner slot 23b of the disc and, whenradiation is passing through that slot, arm 28 of the breaker is incontact with pole at of the breaker. During the other half of therevolution of the chopper disc, arm 28 is in contact with pole b of thebreaker. Breaker B is 90 out of phase with breaker A and is synchronizedwith the outer slot 23a in the disc. Accordin ly, when radiation ispassing through the outer slot, arm 29 of breaker B is in contact withpole at of the breaker, and, for the second half of the revolution ofthe chopper disc, arm 29 is in contact with pole b of the breaker.

The signal reaching each breaker is made up of two components producedby radiation passing through the respective slots. The components are 90out of phase, as are also the breakers. Accordingly, one component ofthe signal reaching a breaker is in phase with the breaker and the otheris 90 out of phase with the breaker. The solid line 3! in Fig. larepresents the in phase components of the signal appearing at pole a ofbreaker B and the broken line (it in Fig. lb

represents the same signal as it appears at pole b of the breaker. Thearm 29 of the breaker is in contact with pole a during the interval-180" and the breaker then transmits that part of the signal representedby the part of line SI beween 0 and 180. When the arm is in contact withpole b of the breaker during the alternate half cycle 180360, thebreaker transmits that part of the signal represented by the part ofline 38 between 180 and 360.

As a result of the action of breaker B as above described, it transmitsa rectified signal I shown in Fig. 4c. The signal I is a D. C. signalproportionate to the in phase radiation. The signal is made up ofalternate sections a, 22 corresponding to intervals when arm 29 was incontact with poles a, b, respectively. As the outer slot 23a in thechopper disc transmits radiation passing through the sample S, thesignal I represents transmission by the sample and has been shown ashaving a value of 50 arbitrary units.

The solid line as represents the out of phase component as it appears atpole a of breaker B and the broken line it represents the same signal asit appears at pole b. As a result of the operation of arm 29, the out ofphase signal is transmitted by the breaker as the signal In shown inFig. 4 Signal I0 is an A. 0. signal having no D. C. component and it is,therefore, completely removed by filter 3|.

The breaker A operates in the same manner as breaker B but 90 out ofphase therewith. As a result, the signal I produced by breaker A inresponse to radiation passing through the outer slot is an A. 0. signaland is not passed by filter 3i but rejected. The signal In produced bybreaker A in response to radiation passing through the inner slot is thein phase signal and it is a D. C. signal, which represents transmissionby the comparison 0 and may be 80 arbitrary units.

The D. C. signals I and I0 passed by the filters 30, 34 to the recordingdevice result in the slider 34 of potentiometer 32 moving its pen tomake a record of the ratio of the signals. Under the corn ditionsassumed with signal I equal to 50 arbitrary units and signal Io equal to80 such units, the ratio of the transmission of the sample for thewavelength used is recorded as 50/8G=62.5%. I he ratio of the absorptionof the sample to that of the comparison is then 37.5% and either ratiomay be recorded by adjustment of the recorder.

The new instrument in the form shown in Figs. 5-7, inclusive may beadvantageously employed in the ultra-violet field and it includes asuitable source S H" receives the radiation and directs it to aspherical mirror 52', which focuses the radiation upon the entrance slitis of a monochromator. The light passing through the slit falls upon afiat mirror 52, from which is passes to an offaxis paraboloid I5 passinga collimated beam through a prism 5 for dispersion. The dispersed beampasses to an adjustable flat mirror H, from which it returns through theprism for further dispersion to the paraboloid. The light is focused bythe paraboloid upon the exit slit I9 and, upon issuing from the slit,falls upon a biprism 46, comprising a pair of right angle prisms lyingin contact with their hypotenuse faces so placed that the two halves ofthe beam issuing at opposite ends of the slit strike the respectiveprisms and are reflected in opposite directions.

The beam reflected by the upper prism 46a (Fig. 6) is periodicallyinterrupted or chopped by a disc 4'! on a driven shaft 48, while thebeam reof ultra-violet radiation. A flat mirror flected by the lowerprism 46b is chopped by;.a disc 49 on shaft 48. The discs havearcuateportions 180 in length removed from their rims with the portionsoifset Accordingly, in each rotation of the shaft, one disc passes itsbeam for a 90 movement of the shaft, while the second beam is cut ofi.In the next 90 of movement of the shaft, both beams are passed by thediscs, and, in the third 90 of movement, the first beam is cut off andthe second is free to pass its disc. In the final 90 of shaft movement,.both beams are cut off.

The beams passing discs 41 and 49 fall, respectively, on focusingmirrors 5t and 5| and the beams travel from the mirrors throughrespective cells 52, 53 and fall upon detector 22'. Oneof the cellscontains a sample of the unknown and the other, the comparison, and thedetector "22' is similar to detector 22, in that it generates a voltagein response to radiation falling thereon. The voltages generated bydetector 22 are utilized in the same manner as shown in Fig. 3 and theinstrument may be employed to make a record of the ratio of theabsorption of the sample to that of the comparison, as above described.

The new instrument is shown in its simplest form in Fig. 8 as includinga source S" of radiation, from which beams pass to cells 54, 55, one ofwhich contains the unknown and the -other, the comparison. The radiationtransmitted through the cells falls upon focusing mirrors 56, 51 and thebeam from mirror at is chopped by a disc 58 similar to disc 4'5, whilethe beam from mirror 57- is chopped by a disc 59 similar to disc 43. Thebeam chopped by disc 58 falls upon the upper part of a biprism essimilar to biprism 46 and is thereby directed upon a detector 22", whilethe beam chopped by disc 59 is directed by the lower part of biprismKill upon detector 22". The voltages generated by the detector are thenamplified and utilized in the same manner as has been described above inconnection with the instrument shown in Fig. 1.

In the instrument of Fig. 8, any appropriate means may be employed forisolating the desired narrow or broad portion of the spectrum of theradiation to be utilized. For this purpose, the source used may be oneof suitable emission characteristics and the detector employed may havethe desired spectral sensitivity. 'Filters may also be employed to cutoff the radiation at upper and lower wave length limits, such filtersbeing well known in the art.

Although the new instrument has been described as including mechanicalrectification, it will be apparent the instrument may include, ifdesired, conventional electrical synchronous rectification from a signalderived from a generator driven by the motor driving the chopper means.

The instrument shown in Fig. 1 is similar to a standard single beaminstrument except for changes in the chopper means, cell, and electronicportion of the instrument. The standard instrument may, accordingly, beconverted to the new form by making the modifications indicated.

In all forms of the new instrument, the order of the components of thesystem between the source and detector is not important, since thechopping means may be placed anywhere in the train and each cell placedat any point where an image of its chopping disc is formed. Thus,instead of placing the disc and cells close together ahead of theentrance slit of the monochromator, as in the Fig. 1 instrument, thecells may be at the entrance to the monochromator and the disc at theexit. In an instrument operating with ultra-violet radiation, it ispreferable to place both the cells and chopping means beyond the exit toth monochromator, as in the instrument shown in Fig. 5.

In the new instrument, the output of breaker A is proportional to theenergy passing through the comparison cell and may be used for theoperation of various auxiliary devices for improving the operation ofthe instrument.

We claim:

1. In an instrument of the type described, the combination of a detectorresponding to radiation falling thereon and producing a signal, meansfor causing radiation of two kinds to fall upon the detectorsuccessively in respective cycles 90 apart, each cycle having an offperiod longer than 90 and the detector producing a signal havingcomponents proportional in amplitude to the respective kinds ofradiation, an amplifier for amplifying said signal, and a rectifyingsystem receiving the amplified signal and including a pair of rectifiersoperating at the frequency of said means, one of the rectifiersoperating approximately in phase with the first component and 90 out ofphase with the second and the other rectifier operating approximately inphase with the second component and 90 out of phase with the first, andfiltering means removing undesired A. C. components from the rectifiedsignals.

2. In an instrument of the type described, the combination of a detectorresponding to radiation falling thereon and producing a signal, meansfor causing radiation of two kinds to fall upon the detector inrespective like cycles ap proximately 90 out of phase, each cycle havingan off period longer than 90 and the detector producing a signal havingcomponents proportional in amplitude to the respective kinds ofradiation, an amplifier for amplifying said signal, and a rectifyingsystem receiving the amplified signal and including a pair of rectifiersoperating at the frequency of said means, one of the rectifiersoperating approximately in phase with the first component and 90 out ofphase with the second and the other rectifier operating approximately inphase with the second component and 90 out of phase with the first, andfiltering means removing undesired A. C. components from the rectifiedsignals.

3. In an instrument of the type described, the combination of a detectorresponding to radiation falling thereon and producing a signal, meansfor causing radiation of two kinds to fall upon the detector inrespective cycles 90 out of phase, each cycle having an on period and anoff period, the on and off periods in each cycle being-longer than 90and the detector producing a signal having components proportional inamplitude to the respective kinds of radiation, an amplifier foramplifying said signal, and a rectifying system receiving the amplifiedsignal and including a pair of rectifiers operating at the frequency ofsaid means, one of the rectifiers operating approximately in phase withthe first component and90 out of phase with the second and the otherrectifier operating approximately in phase with th second component and90 out of phase with the first, and filtering means removing undesiredA. C. components from the rectified signals.

4. In an instrument of the type described, the combination of a detectorresponding to radiation fallin" thereon and producing a signal, meansfor causing radiation of two kinds to fall upon the detectorsuccessively in respective cycles out of phase, each cycle having an offperiod longer than 90 and the detector producing a signal havingcomponents proportional in amplitude to the respective kinds ofradiation, an amplifier for amplifying said signal, a rectifying systemreceiving the amplified signal and including a pair of rectifiersoperating at the frequency of said means, one of the rectifiersoperating approximately in phase with the first component and 90 out ofphase with the second and the other rectifier operating approximately inphase with the second component and 90 out of phase with the first, andfiltering means removing undesired A. C. components from the rectifiersignals, and recording means receiving the rectified signals and makinga record in respons thereto.

5. In an instrument of the type described, the combination of a detectorresponding to radiation falling thereon and producing a signal, meansfor causing radiation of two kinds to fall upon the detectorsuccessively in respective cycles 98 out of phase, each cycle having anoff period longer than 90 and the detector producing a signal havingcomponents proportional in amplitude to the respective kinds ofradiation, an amplifier for amplifying said signal, and a rectifyingsystem receiving the amplified signal and including a pair of mechanicalbreakers operating at the frequency of said means, one of the breakersoperating approximately in phase with the first component and 90 out ofphase with the second and the other breaker operating approximately inphase with the second component and 90 out of phase with the first, andfiltering means removing undesired A. C. components from the rectifiedsignals.

6. In an instrument of the type described, the combination of a detectorresponding to radiation falling thereon and producing a signal, meansfor causing a pair of beams of respective different kinds of radiationto fall upon the detector, means for chopping the beams in respectivecycles 90 out of phase, each cycle having an off period longer than 90and the detector producing a signal having components proportional inamplitude to the radiation in the respective beams, an amplifier foramplifying the signal, and a rectifying system receiving the amplifiedsignal and including a pair of rectifiers operating at the frequency ofthe chopping means, one of th rectifier-s operating approximately inphase with the first component and 90 out of phase with the second andthe other rectifier operating approximately in phase with the secondcomponent and 90 out of phase with the first, and filtering meansremoving undesired A. C. components from the rectified signals.

7. In an instrument of the type described, the combination of a detectorresponding to radiation falling thereon and producing a signal, means,including monochromating means, for causing narrow bands of Wavelengthsof radiation of two kinds to fall upon the detector out of phase inrespectiv cycles 90 out of phase, each cycle having an off period longerthan 90 and the detector producing a signal having componentsproportional in amplitude to the respective bands of radiation, anamplifier for amplifying the signal, and a rectifying system receivingthe amplified signal and including a pair of rectifiers operating at thefrequency of said means first-named, one of the rectifiers operatingapproximately in phase with the first component 9 and 90 out of phasewith the second and the other rectifier operating approximately in phasewith the second component and 90 out of phase with the first, andfiltering means removing undesired C. components from the rectifiedsignals.

8. In an instrument of combine. ion of means for emitting radiation, adetector receiving radiation from said means and producing a signal inresponse thereto, a pair of cells lying in the path or radiationtraveling from said emitting means to the detector and offset laterally,the cells being adapted to contain a sample to be analyzed and acomparison sample, respectively, a monochromator receiving radiationtraveling from the emitting means toward the detector and passing anarrow band of wavelengths of said radiation, chopping means between theemitting means and detector operating cy clically to cut oil the 1" tionand to pass radiation transmitted by the iespective cells approxi mately90 out of phase, whereby, in each cycle of the chopping means, thedetector produces a signal having components proportional in amplitudeto radiation. transmitted by the respective cells, an amplifier foramplifying the signal, and a rectifying system receiving the amplifiedsignal and including a pair of rectifiers operating at the frequency ofthe chopping means, one of the rectifiers operating approximately inphase with the first component and 90 out of phase with the second andthe other rectifier operating approximately in phase with the secondcomponent and 96 out of phase with the first, and filtering meansremoving undesired A. components from the rectified signals.

9. In a spectrophotometer, the combination of a source of radiation, adetector responding to radiation falling thereon and producing a signal,a pair of cells adapted to contain samples to be compared and lying inthe path of radiation traveling from the source to the detector, thecells being offset laterally, means between the source and the detectorfor chopping the radiation, said chopping means operating in cycles,

the type described, the

each of which includes the steps of cutting off radiation and passingradiation transmitted by the respective cells, whereby, in each cycle ofthe chopping means, the detector produces a signal having componentsproportional in amplitude to radiation transmitted by the respectivecells, an amplifier for amplifying the signal, and a rectifying systemreceiving the amplified signal including a pair of rectifiers operatingat the frequency of the chopping means, one

of the rectifiers operating approximately in phase with the firstcomponent and 99 out of phase with the second and the other rectifieroperating approximately in. phase with the second component and 90 outof phase with the first, and filtering means removing undesired A. C.components from the rectified signals.

10. In a spectrophotometer, the combination of a source of radiation, adetector responding to radiation falling thereon and producing a signal,a pair of cells adapted to contain samples to be compared and lying inthe path of rad ation traveling from the source to the detector, thecells being offset laterally, means between the source and the detectorfor chopping the radiation, said chopping means operating in cycles,each or which includes the steps of cutting ofi radiation and passingradiation transmitted by the respective cells approximately 90 out ofphase, whereby, in each cycle of the chopping means, the detectorproduces a signal having til iii)

ill

components proportional in amplitude to radiation transmitted by therespective cells, an amplifier for amplifying the signal, and arectifying system receiving the amplified signal and including a pair ofrectifiers operating at the frequency of the chopping means, one of therectifiers operating approximately in phase with the first component and99 out of phase with the second and the other rectifier operatingapproximately in phase with the second component and out of phase withthe first, and filtering means removing undesired C. components from therectified signals.

11. In an instrument of the type described, the combination of a pair ofcells adapted to contain samples of a standard and an unknown,respectively, a detector responsive to radiation falling thereon, means,including a source of radiation, for causing separate beams of radiationto travel along diiferent paths through respective cells and fall uponthe detector, and means for interrupting the beams in respective cycles90 out of phase, each cycle having an off period longer than 90.

12. In an instrument of the combination or a pair type described, the ofcells adapted to contain samples of a standard and an unknown,respectively, means, including a source of radiation, for causingseparate beams of radiation to travel along different paths throughrespective cells and to be combined into a single beam, a monochromatorreceiving said single beam and dispersing said beam, said monochromatorhaving an exit slit for passing a narrow band of wavelengths of saiddispersed beam, a detector receiving radiation issuing through the exitslit and responding to radiation, and means for interrupting saidseparate beams in respective cycles 90 apart, each cycle having an of!"period longer than 90.

13. An instrument of the type described, the combination of a pair ofcells adapted to contain samples of a standard and an unknown,respectively, a detector responsive to radiation falling thereon, means,including a source of radiation, for causing separate beams of radiationto travel along different paths through respective cells and fall uponthe detector, and means for interrupting the beams in respective cycles90 out of phase, each cycle having an on period and an off period bothlonger than 90.

14. In an instrument of the type described, the combination of a pair ofcells adapted to contain samples of a standard and an unknown,respectively, means, including a source of radiation, for causingseparate beams of radiation to travel along different paths throughrespective cells and be combined into a single beam, a monochromatorreceiving said single beam and dispersing said beam, said monochromatorhaving an exit slit for passing a narrow band of wavelengths of thedispersed beam, a detector receiving radiation issuing through said exitslit and responding to said radiation, and means for interrupting theseparate beams in respective cycles 90 out of phase, each cycle havingan on period and an on" period both longer than 90.

References Cited. in the file of this patent UNITED STATES PATENTSNumber Name Date 1,999,023 Sharp et a1 Apr. 23, 1935 2,287,808 LehdeJune 30, 1942 2,350,734 Hood Oct, 10, 1944 (Gther references onfollowing page) Number 11 UNITED STATES PATENTS Name Date Major et a1.Feb. 26, 1946 Stearns Apr. 6, 1948 Thomson June 8, 1948 White Oct. 4,1949 Meyer Apr, 4, 1950 Vassy Nov. 7, 1950 Jamison et a1 Apr. 3, 1951Vossberg Apr. 17, 1951 FOREIGN PATENTS Country Date Number Great BritainJan. 16, 1947 12 OTHER REFERENCES Kivenson et al., An Infra-Red.Chopped-Radiation Analyzer, pages 1086 through 1091, vol. 38 of Journalof the Optical Society of America, December 1949.

Kivenson, Some Considerations in the Design of Double-Beam Analyzers forIndustrial Control, pages 112 through 118, Journal of the OpticalSociety of America, Vol. 49, February 1950.

